Electric cookers supply heat to utensils placed on their heater units by means of heat transmitted to the utensil by convection, conduction and radiation. Of these, conduction and radiation are dominant radiant heat from the heater unit passing directly to the utensil and heat being conducted to the utensil by contact of the utensil with the heater unit. The amount of heat conducted to the utensil is dependent, of course, on the degree of contact between the utensil and the heater unit. In the case of a glass ceramic top cooker, the contact is with the smooth glass ceramic plate which is heated by the heater unit.
A glass ceramic top cooker is one in which a smooth plate of glass ceramic overlies one or more generally circular electric heater elements supported on a layer of thermal and electrical insulating material such that the or each heater element is spaced from the underside of the glass ceramic plate. In use, a utensil placed on the glass ceramic plate above a heater element is heated by the transmission of heat from the heater element to and through the glass ceramic plate by air convection, conduction and infra-red radiation. Such heater elements are referred to as radiant heaters. The insulating material substantially prevents heat being transmitted away from the heater elements except towards the glass ceramic plate and, because the preferred materials for the plate are essentially thermally non-conductive, only areas of the plate which are directly exposed to the heater element will be heated. In order to prevent heat being transmitted to parts of the glass ceramic plate not covered by a utensil placed directly above a heater element, a peripheral wall of thermal insulating material is also normally provided around the heater element.
In addition, a thermal cut-out device extends above the heater element in order to protect both the heater element and the glass ceramic plate against overheating. The thermal cut-out device comprises a differential expansion probe which extends above the heating element and a snap-acting switch located outside the peripheral wall so as to disconnect the heating element from its electrical supply in order to prevent excessive temperatures which can be hazardous to the user and can cause damage to or discolouration of the glass ceramic cooking surface.